Henry David Thoreau would have you believe that he lived off his bean patch by Walden Pond while all the time he was dining with Ralph Waldo Emerson and making almost daily trips home from his cabin in the woods to see his mother.

Therein lies an anomaly, and it is the business of the biographer to find out what Thoreau was up to whether he really found solace beside the pond at Walden or whether he could face up to society in downtown Concord, Mass.

This question was posed by Leon Edel, the Pulitzer Prize-winning biographer of Henry James, at the opening of a two-day "Symposium of the Art of Biography" at the National Portrait Gallery yesterday.Edel said the answer to such anomalies was like finding "the figure under the carpet," the title of his talk.

The symposium is the last of a series of events marking the 10th anniversary of the gallery. Officials said biography was chosen as a topic because of the obvious relationship between pictures of people and stories of their lives. Moreover, they pointed out that a recent study by the Library of Congress showed that biography was increasingly popular on people's reading lists.

Doris Kearns, author of a famous study of Lyndon B. Johnson, spoke at last night's session.

Other scheduled speakers are Barbara Tuchman, author of "The Guns of August" and "Stillwell and the American Executive in China," for which she won Pulitzer Prizes, and Justin Kaplan, who won a Pulitzer in 1967 for his "Mr. Clemens and Mark Twain."

Edel told his audience of fellow biographers that a biographer "is as much a storyteller as a novelist." He said a biographer is "a storyteller who is not allowed to invent his facts but is allowed to invent his form."

So it is proper, he said, to wonder what really motivated Thoreau to live in the woods and it is proper to wonder whether Ernest Hemingway was the self-confident he-man he let on to be. Otherwise, why did he punch Max Eastman when Eastman said Hemingway made too much of the hair on his own chest?

In short, biographers should try to find out what their subjects really are. That may be why biography is so popular.

Edel described his own work on Henry James as a "relatively short" one. In the question period, someone asked if a five-volume work could properly be described as short.

Well, said Edel, it was short in terms of the amount of mateial he had. Moreover, it's available in an uncut two-volume version in Britain.

He said he never tired of reading the works of Henry James.

"Of couse," he added, "I'm sure many people would have the opposite experience with him."